Texas Instruments SN74LS540DB

Overview

SN74LS540DB from Texas Instruments is an octal buffer/line driver with 3-state outputs, designed for bidirectional data bus interface in TTL-based digital systems. It features active-low output-enable (OE̅), 8-bit non-inverting logic, ±24 mA output drive capability, and operates over 0°C to 70°C. It is commonly used in microprocessor address/data bus buffering and memory I/O expansion.

For engineers reviewing the SN74LS540DB datasheet, SN74LS540DB pinout, SN74LS540DB application, or SN74LS540DB equivalent, this page delivers verified electrical specs, SSOP-20 package details, real-world use cases, and validated alternative parts for legacy system maintenance and industrial control design.

Technical Context

The SN74LS540DB implements standard TTL-compatible input thresholds (V_IL = 0.8 V max, V_IH = 2.0 V min) and provides high-current 3-state outputs capable of sinking 24 mA or sourcing 15 mA. Its propagation delay is 15 ns typical at V_CC = 5 V and T_A = 25°C, supporting reliable operation in synchronous bus architectures.

It uses bipolar transistor-transistor logic (TTL) process technology, ensuring compatibility with legacy LS-family logic families. The device includes internal clamp diodes on all inputs and outputs, enabling robust ESD protection per MIL-STD-883 Class 3B (2 kV HBM).

Key Specifications

Pinout & Package

SN74LS540DB is housed in a 20-pin Small Outline Shrink Plastic (SSOP) package (drawing DB), measuring 7.5 mm × 5.6 mm with 0.65 mm lead pitch and maximum height of 2.0 mm. Pin 1 is located at the top-left corner adjacent to the index notch.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal 3-state buffer applications.

Compared with SN74LS540DB, SN74LS240N offers inversion and dual-OE flexibility but adds polarity constraints, while SN74LS541DBR delivers drop-in functional equivalence with stronger long-term supply assurance in the same SSOP package.

Availability

SN74LS540DB is available at Aetrix Electronics and suitable for microprocessor bus interface, memory address latching, industrial I/O expansion, and legacy test equipment control requiring stable component supply across extended product lifecycles.

Supply support for SN74LS540DB includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a global semiconductor leader founded in 1930, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74LS540DB belongs to TI's legacy 74LS logic family, engineered for reliability and interoperability in commercial-grade digital systems requiring proven TTL performance and long-term obsolescence mitigation.

FAQ

What is the function of SN74LS540DB in a digital system?

The SN74LS540DB functions as an octal non-inverting buffer with 3-state outputs, enabling controlled data flow on shared buses. It isolates upstream logic from bus loading, drives multiple TTL inputs, and supports bidirectional communication when paired with complementary devices. Its active-low output-enable (OE̅) allows precise timing control in synchronous architectures, making SN74LS540DB essential for microprocessor address/data bus management and legacy system interfacing.

Does SN74LS540DB support 3.3 V logic levels?

No, SN74LS540DB is a 5 V-only TTL device with input thresholds defined for 5 V operation (V_IL ≤ 0.8 V, V_IH ≥ 2.0 V). It does not guarantee correct switching with 3.3 V CMOS logic levels and lacks 5 V tolerant inputs. Interfacing with 3.3 V systems requires level-shifting circuitry or replacement with a 3.3 V–compatible buffer such as the SN74LVC244A. SN74LS540DB must be powered from a regulated 5 V supply to meet its specified timing and drive performance.

What is the maximum clock frequency SN74LS540DB can support?

SN74LS540DB does not operate on a clock signal-it is combinational logic with propagation delay rather than a clocked device. Its 15 ns typical t_PD supports reliable operation in bus systems with cycle times ≥30 ns (i.e., ≤33 MHz effective rate), assuming proper setup/hold margins and load conditions. System-level maximum frequency depends on trace length, capacitive loading, and driver strength; SN74LS540DB is routinely used in 8–12 MHz Z80 and 8085 systems without timing violation.

Is SN74LS540DB RoHS compliant?

Yes, SN74LS540DBR.A (the active orderable variant matching SN74LS540DB) is RoHS compliant, with NiPdAu (NIPDAU) lead finish and MSL Level-1 rating for unlimited floor life at ≤30°C/85% RH. It meets EU Directive 2011/65/EU requirements and carries no exemptions. The "No" RoHS status shown for military-grade variants (e.g., SNJ54LS540J) does not apply to SN74LS540DB, which is a commercial catalog part manufactured post-RoHS transition.

Can SN74LS540DB replace SN74LS240 in existing designs?

SN74LS540DB cannot directly replace SN74LS240 without circuit modification because SN74LS240 provides inverting outputs and dual active-low OE inputs, whereas SN74LS540DB offers non-inverting outputs and a single OE̅. Swapping them would invert data paths and alter enable logic behavior. However, SN74LS540DB may substitute for SN74LS241 (non-inverting version of 240) or SN74LS541, both of which share identical functionality and pinout. Always verify signal polarity and OE configuration before substitution.